1. Field of the Invention
The present invention relates to a sol-gel method for producing aluminum effect pigments coated with silicon oxide, preferably SiO2, with a corrosion-resistant coating for waterborne paints.
2. Description of Related Art
The meteoric rise in the use of eco-friendly waterborne paints in the automobile segment and in industrial coatings has necessitated the development of corrosion-protected metallic effect pigments. In the waterborne paints, which are given a basic formulation, the widespread aluminum effect pigments, in particular, exhibit a propensity toward oxidation, which produces hydrogen and leads to oxidization of the aluminum effect pigment. This oxidization results in a loss of specular gloss, such loss also being referred to as graying.
Very effectively corrosion-protected aluminum effect pigments are produced by the chromating process (EP 0 259 592) and are available from Eckart under the tradename Hydrolux®. These corrosion-protected aluminum effect pigments are notable for excellent gassing stability and outstanding opacity. Opacity, also called hiding power, is the surface area hidden per unit weight of pigment. The opacity of the chromated aluminum effect pigments is more particularly comparable with the opacity of the aluminum effect pigments prior to chromating.
A disadvantage is that the chromated aluminum effect pigments contain chromium compounds. Although chromated aluminum effect pigments do not contain any detectable amounts of the toxic Cr(IV), they are nevertheless not advantageous environmentally in view of the heavy metal content.
For this reason, SiO2-coated aluminum or gold bronze effect pigments were developed. Coating with SiO2 is accomplished preferably using the sol-gel process, in which, first of all, the aluminum or gold bronze effect pigments undergo sol-gel encapsulation, and subsequently a silicon dioxide coating is formed. The SiO2-coated aluminum or gold bronze effect pigments have a high corrosion resistance, since the barrier effect of, the silicon dioxide coating prevents the migration of water or other corrosive substances at the pigment surface.
SiO2 coating takes place via a gentle, eco-friendly sol-gel process which is catalyzed by bases (A. Kiehl, K. Greiwe Progr. in org. Coatings 37 (1999), 179-183). Commercially available metallic effect pigments SiO2 coated using sol-gel processes are Hydrolan® (aluminum effect pigments) and Dorolan® (gold bronze effect pigments) from Eckart. Other commercially available SiO2-coated aluminum effect pigments are, for example, Emeral® from Toyo, Japan, Aquamet® from Schlenk, Germany, and Silbercote® from Silberline, USA.
The gassing stability possessed by SiO2-coated aluminum effect pigments is generally sufficient. By sufficient gassing stability is meant that under the influence of water there is generally no substantial evolution of hydrogen, since the aluminum is protected relatively effectively against attack by water. The gassing stability, however, is also dependent on the ambient conditions to which the aluminum effect pigment is exposed.
Occasionally, in the case of metallic effect pigments having a very fine particle-size band and a correspondingly large specific surface area, i.e. surface area per unit weight of metallic effect pigment, unwanted fluctuations may occur in the gassing stability. The opacity of the SiO2-coated aluminum effect pigments known from the prior art, however, decreases markedly, as compared with the starting material and in comparison to chromated pigments.
For the coating of substrates with silicon dioxide there are two methods described in the prior art as being essential. The first method is the utilization of alkali metal silicates, which are converted into silanols by catalyzed hydrolysis before subsequently coalescing to form an inorganic network (R. K. Iler et al U.S. Pat. No. 2,885,366, 1959; R. K. Iler “The Chemistry of Silica”, 1979).
The second method is the utilization of the sol-gel process, starting from alkoxysilanes, which are reacted under catalysis with water to form silanol and alcohol. In the conventional sol-gel coating of aluminum effect pigments using alkoxysilanes, the starting pigment in powder form is dispersed in an alcoholic phase and then the alkoxysilanes, water, and at least one basic or acidic catalyst are added with accompanying supply of heat.
The hydrolysis of tetraethoxysilane compounds is accompanied by formation of silanol structures of the composition Si(OH)4-y(OCH2CH3)y (y=0-3), which are able to enter into polycondensation reactions. In the course of the reaction, a compact network of silicon dioxide develops on the surface of the pigment and completely encapsulates the pigment particles. Furthermore, the silicon dioxide coating freshly precipitated onto the pigment surface can be specifically subjected to further surface modifications. For example, silanes having at least one nonhydrolyzable substituent, examples being alkylsilanes, can be added after the application of the SiO2 coating and can be hydrolyzed in situ, with the silanes having at least one non-hydrolyzable substituent being firmly anchored, via further condensation reactions, to and on the silicon dioxide layer on the pigment surface. The filtercake which is obtained after cooling and suction removal of the solution can be dried under reduced pressure and supplied for the use as intended.
U.S. Pat. No. 2,885,366 A discloses a basic method for producing a product surface-stabilized by with metal oxides, it also being possible for this product to consist of SiO2-coated metallic effect pigments.
A fundamental method for producing effect pigments coated with reactive orientation assistants—using a basic catalyst—is described in DE 198 20 112 A1.
Waterborne basecoat materials comprising SiO2-coated aluminum effect pigments are disclosed in EP 1 332 714 A1.
WO 2004/026268 A2 as well discloses a method for producing a corrosion-stable metallic effect pigment for a cosmetic product, which involves providing the aluminum core with an SiO2 coating by means of a sol-gel process using suitable catalysts. The catalysis itself is not described in any more detail in that patent application.
EP 1 756 234 B1 relates to a method for producing an aqueous coating composition which comprises at least one water-compatible, film-forming agent and aluminum effect pigments provided with at least one inorganic corrosion protection layer. These aluminum effect pigments have at least one SiO2 layer produced by a sol-gel process. No details relating to the stabilization with respect to corrosion are evident from that patent.
The U.S. Pat. No. 7,419,538 B2 discloses a process for producing platelet-shaped aluminum effect pigments which are pretreated with phosphoric acid and/or boric acid and are subsequently provided with an SiO2 layer by a sol-gel process in water. With this method, a basic catalyst is used in the course of the coating operation.
EP 1 619 222 A1 relates to a process for producing an aluminum effect pigment provided with a molybdenum coating and/or SiO2 coating and intended for water-based inks, using organic bases, such as ethanolamine, for example, or organic or inorganic acids, such as sulfuric acid or oxalic acid, for example, as catalysts.
EP 1 953 195 A discloses a method for stabilizing aluminum effect pigments by applying a multilayer coating. In that case the pigment, pretreated with phosphorus compounds or molybdenum compounds, is encapsulated with silicon dioxide by an acidically or basically catalyzed sol-gel synthesis, and is subsequently clad in a separate step with an organic polymer matrix.
WO 03/095564 A1 relates to a process for producing goniochromatic luster pigments having a coating which displays interference colors, with a polar organic solvent being incorporated into the coating. To produce these goniochromatic luster pigments, the pigment particles, such as, for example, corrosion-stabilized aluminum effect pigments, are first coated with a dielectric layer of low refractive index, such as silicon dioxide, for example, and are subsequently provided with a reflective coating. Wet-chemical coating of the aluminum effect pigments with silicon dioxide takes place in the basic pH range.
Lastly, JP 2004124069 A discloses a process for producing silicon dioxide-coated aluminum effect pigments for water-based application systems, using basic catalysts.
The known processes for the SiO2 coating of metallic effect pigments have the disadvantage, however, that they do not always—especially in the case of very fine metallic effect pigments—ensure sufficient corrosion stability, more particularly gassing stability, of the metallic effect pigments obtainable therewith. Moreover, there is usually a distinct loss of opacity as a result of the SiO2 coating.